a) Field of the Invention
The invention relates to a safety device for a moving system with a drive in which a pneumatically operated piston, by linear movement in a cylinder, moves a transported object past a stationary part in at least one direction.
b) Description of the Related Art
In the fabrication of integrated circuits, because of cleanroom b requirements the substrates to be machined are usually accommodated in pods or transport containers which are transported between different machining steps to the individual stations. Loading and unloading in the stations takes place primarily via closable openings or locks. Due to the fact that the edges or walls of the openings past which the transport containers are moved present hazards for the operating personnel, steps must be taken to prevent the risk of injury. Usually, safety devices are employed which stop all transporting movements and possibly initiate a reversal of the movement when a hazardous situation occurs.
While known light curtains or light barriers provide protection around permissible openings to prevent engagement therein, these safety measures must be switched off during the period in which moving objects pass through. Otherwise, an interruption of movement would result. It is disadvantageous that the protective function is canceled during this through-period.
U.S. Pat. No. 5,575,372 discloses a device of the type mentioned above when two parts moving past one another approach each other, wherein a safety yoke is fastened to one of the parts and is displaceable relative to the part. The safety yoke contains electrically conducting connection elements which connect printed circuits on the part in a reference position of the safety yoke to produce a closed circuit. If an object is pushed against the yoke, a switching signal is generated by the interruption of the circuit so that the forward movement is terminated. It is disadvantageous that the yoke always acts in only one direction. In order to provide security for a forward and backward movement, means acting in both directions are required. It is often not feasible to provide an additional yoke for reasons of space. It is also usually very complicated to ensure all-around protection for moving parts.
It is the primary object of the invention to generate a switching signal at every point of a through-opening when an obstacle is located between the moving object and the edge of the through-opening, regardless of the movement direction of the object and the geometrical construction of the through-opening.
This object is met through a safety device for a moving system with a drive in which a pneumatically operated piston, by linear movement in a cylinder, moves a transported object past a stationary part in at least one direction. According to the invention, the cylinder has, at its ends, devices for measuring the pressure of the air flowing out of the cylinder in the movement direction of the piston, wherein a drop in the otherwise substantially constant pressure below a threshold value serves as a switching signal at least for switching off the system.
Additional devices for measuring pressure by which at least one additional threshold value can be adjusted or which permit analog monitoring of pressure can advantageously be provided.
Further, it is possible to combine the measurement of pressure with a measurement of the spatial position and/or the velocity of the moving system.
Valves with either a closed or open center position can be used for supplying compressed air.
The solution to the above-stated problem has further advantages consisting in that no additional mechanical parts such as yokes, frames, switching strips, sensor arrangements or receiver arrangements are required at the edges of the opening. The switching signal is generated during inward and outward movement without additional mechanical means when an object arrives between the moving system and the boundary of the opening. Accordingly, the solution is executed in a simple and economical manner.
The adjustment of the safety device can vary within a wide range (adjustment of force). The evaluation of-the measured pressure can be carried out by means of a simple switch-point evaluation or by analog area evaluation.
In combination with a path measurement system, different force adjustments, e.g., to the travel area at points of various hazards, can be adjusted, selected and evaluated.
Depending on the pressure measuring device, the switching signal can act directly to switch off the drive, or evaluation is carried out first via electronics or a control unit. Accordingly, it is possible not only to switch off the drive but also to carry out a reversal of the direction of movement in connection with hardware and software.
The invention will be explained more fully in the following with reference to the schematic drawing.